Optical and Electrical Properties of GaN-Based Light Emitting Diodes

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 47, NO. 7, JULY 2011

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Optical and Electrical Properties of GaN-Based Light Emitting Diodes Grown on Micro- and Nano-Scale Patterned Si Substrate Ching-Hsueh Chiu, Chien-Chung Lin, Dong-Mei Deng, Da-Wei Lin, Jin-Chai Li, Zhen-Yu Li, Gia-Wei Shu, Tien-Chang Lu, Ji-Lin Shen, Hao-Chung Kuo, Senior Member, IEEE, and Kei-May Lau, Fellow, IEEE

Abstract— We investigate the optical and electrical characteristics of the GaN-based light emitting diodes (LEDs) grown on micro- and nano-scale patterned silicon substrate (MPLEDs and NPLEDs). The transmission electron microscopy images reveal the suppression of threading dislocation density in InGaN/GaN structure on nano-pattern substrate due to nano-scale epitaxial lateral overgrowth. The plan-view and cross-section cathodo luminescence mappings show less defective and more homogeneous active quantum-well region growth on nano-porous substrates. From temperature-dependent photoluminescence (PL) and low temperature time-resolved PL measurement, NPLEDs have better carrier confinement and higher radiative recombination rate than MPLEDs. In terms of device performance, NPLEDs exhibit smaller electroluminescence peak wavelength blue shift, lower reverse leakage current and decrease in efficiency droop when compared with the MPLEDs. These results suggest the feasibility of using NPSi for the growth of high quality and power LEDs on Si substrates. Index Terms— Light emitting diodes, metal-organic chemical vapor deposition, nano-scale epitaxial lateral overgrowth, silicon substrate.

Manuscript received December 15, 2010; revised February 1, 2011; accepted February 7, 2011. Date of current version May 13, 2011. This work was supported in part by the National Science Council of Taiwan, under Grant NSC 98-3114-E-009-002-CC2. C.-H. Chiu, D.-W. Lin, J.-C. Li, Z.-Y. Li, T.-C. Lu, and H.-C. Kuo are with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: [email protected]; [email protected]; jinchaili@ xmu.edu.cn; [email protected]; [email protected]; hckuo@ faculty.nctu.edu.tw). C. C. Lin is with the Department of Photonics and Institute of ElectroOptical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. He is also with the Institute of Photonic Systems, College of Photonics, National Chiao Tung University, Tainan 71150, Taiwan (e-mail: [email protected]). D.-M. Deng is with the Department of Photonics and Institute of ElectroOptical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. He is also with the Photonics Technology Center, Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: [email protected]). K.-M. Lau is with the Photonics Technology Center, Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: [email protected]). G.-W. Shu and J.-L. Shen are with the Department of Physics, Chung Yuan Christian University, Chung-Li 32023, Taiwan (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2011.2114640

I. I NTRODUCTION

T

HE wide band gap GaN-based semiconductors have received enormous attention for various applications, such as short-haul optical communication, traffic and signal lights, back lights for liquid-crystal displays, and indoor/outdoor lightings. Typically, GaN-based light emitting diodes (LEDs) were grown on sapphire or SiC substrate by heteroepitaxial techniques in a metal-organic chemical vapor deposition (MOCVD) system [1]–[3]. However, the low thermal and electrical conductivities make sapphire less perfect as a substrate for the GaN epilayers, meanwhile the high price and mechanical defects hinder SiC substrate’s acceptability in the LED market. Silicon has been considered as an alternative substrate material due to its low manufacturing cost, availability of large size wafers, and good thermal and electrical conductivities. Thus, many efforts have been dedicated to the realization of GaN based LEDs on Si substrates [4]–[8]. Even though good progress has been made, there are still several problems when using Si substrate for GaN epitaxial layers. The large lattice mismatch between GaN and Si (almost 17%) leads to high threading dislocation densities (TDDs) (around 108 − 1010 cm−2 ) in the subsequent GaN epilayers. The other major problem is the thermal expansion coefficient difference (56%) between two materials, which induces a high tensile stress during the thermal cycling in MOCVD and often results in cracks and damages of epilayers [9]. To reduce the density of cracks and threading dislocations of GaN grown on Si, a number of approaches have been reported, such as using AlN multilayer combined with graded AlGaN layer as buffer [10], epitaxial lateral overgrowth of GaN on micro-patterned Si [11], and nano heteroepitaxial (NHE) lateral overgrowth of GaN on nanopore array Si [12], etc.. These methods effectively reduce the tensile stress and thus the crystal quality of GaN was greatly improved. Recently, our co-workers reported fabrication of GaN-based device structure on a nanoscale patterned silicon substrate [13] that shows significant improvement on reduction of TDDs, surface morphology and light emission. In the mean time, the optical and electrical properties of InGaN/GaN MQWs grown on these patterned silicon substrates have not been fully studied yet. In this paper, we examine various optical and electrical characteristics of GaN based LEDs grown on micro and nano-scale patterned

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Schematic diagram for preparing the porous Si substrate. TABLE I S UBSTRATES U SED FOR G ROWING GaN L AYERS Substrate A-1 A-2 A-3

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Si substrates (MPLEDs and NPLEDs), and the experimental results can lead us to believe that NPLED is in general superior to its micro-scale counterpart. II. E XPERIMENTS The micro-scale pattern Si (MPSi) substrate was prepared into 340μm × 340μm square islands on a 2-inch silicon substrate. These islands are separated by 3μm deep and 20μm wide trenches, in the 110 and 112 directions, and were patterned by an STS inductively coupled plasma-reactive ion etching (ICP-RIE) system on 2-inch Si substrate. The self-ordered anodized aluminum oxide (AAO) procedures are depicted in Fig. 1. Firstly, a 500 Å thick SiO2 film acting as the isolation layer was formed by thermal oxidation on a 2-inch Si (111) substrate. After that, 500 Å Ti and 3500 Å Al were deposited on it one by one using an AST electronbeam evaporator. The Ti improved the adhesion of the Al layer and promoted the uniformity of the porous alumina in the anodization step. The procedure of anodization can be summarized in the following four steps [13], [14]: first we deposit a non-conductive oxide layer and submerge the wafer into the electrolyte. Second, due to the inherent roughness, the electric field will locally concentrate at the high curvature points (Step 2). This local high field leads to a field-enhanced or/and temperature-enhanced dissolution of formed oxide, and thus, pores grow with the gradually dissolved alumina (Step 3). When the formation and dissolving of alumina reach an equilibrium state, a stable growth of pores can be realized, as shown in step 4. Finally after the alumina nano-particles were formed, the oxide and then the underneath semiconductor layer can be removed by generic etching process (as shown in Fig. 2). In general, there are some parameters influencing the selfordered anodized aluminum oxide (AAO), such as the anodic voltage, type and concentration of electrolyte, temperature, etc. [15] Among these factors, the anodic voltage is one of the

most important factors for adjusting inter-pore distance. It is reported that the inter-pore distance was proportional to the anodic voltage, and could get the following relation [14], 2.5(nm/V) U ≤ Dint ≤ 2.8(nm/V) U

(1)

[16], [17] where Dint is the inter-pore distance, and U is the applied voltage. The aforementioned equation (1) can served as a baseline for the process. However, in different material system, there should be one or more optimal conditions for the subsequent material quality. In this work, several designs were carried out to find out the optimized processes, and we summarize the physical characteristics in Table 1. Their outcomes of epitaxial layer quality can be visually distinguished from Fig. 3(a) to 3(c). When the size of the pore is too large, the coalescence of GaN layer can not be fully developed due to large pore diameter to mesa width ratio. If the depth of the pore is too large, the surface morphology will also be affected badly. We choose the condition in Fig. 3(c) as the final template for NPLED because it can deliver the best quality of the material. In addition to the anodic voltage and timing control, the common condition of anodization electrolyte was at 6 °C in 0.3M phosphoric acid for 30min. After anodization, selfassembled AAO nano arrays were uniformly distributed on the Si surface. By ICP etching, the AAO pattern was transferred to the Si substrate. The AAO mask was then removed by wet etching. When all AAO steps were carried out successfully, nanopore arrays were uniformly distributed on the entire 2-inch Si substrate with an average nano-pore diameter of 150 nm, interpore distance of 120 nm, and an etched depth of 250 nm. In the next step, LED structures with In0.08Ga0.92 N/GaN MQWs were grown on this nano-patterned substrate by MOCVD in

CHIU et al.: OPTICAL AND ELECTRICAL PROPERTIES OF GaN-BASED LEDs GROWN ON Si SUBSTRATE

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Fig. 3. Opticalmicroscope of GaN layer grown on substrate (a), (b), and (c). P-GaN InGaN/GaN MQWsx5 n-GaN AlN/AlGaN u-GaN AlGaN AlN MPSi

NPSi

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Fig. 4. Schematic of GaN-based LED structures grown on (a) MPSi and (b) NPSi.

an Aixtron 2000HT system. The epitaxial structure of the GaN-based LED overgrowth on MPSi and NPSi substrate is depicted in Fig. 4. Detailed substrate preparation and growth procedure for LED on MPSi and NPSi substrate were reported elsewhere [18], [19]. After the InGaN/GaN structures were grown, we performed standard LED lithographic process, metallization, and etch procedure in order to define device mesa and make p/n contacts of the LED layers. Once the device fabrication is finished, we engaged four different types of measurements: cathodo luminescence (CL), photoluminescence (PL), timeresolved photoluminescence (TRPL) and electroluminescence (EL). The spatially resolved CL imaging was obtained by scanning electron microscope (JEOL-7000F SEM system) with a fixed viewing scale. The temperature dependent PL measurements were done by a 325 nm He-Cd laser at 35 mW excitation power. Low temperature TRPL measurements were performed at 10 K using time-correlated single-photon counting and a pulsed GaN diode laser operating at a wavelength of 396 nm as the excitation source. In the EL measurement system, the current source is Kiethley 238, and the best measurement resolution at 1 nA injection could reach 10 fA with a accuracy of 0.3%. We can perform a serious of current-voltage measurement and data storage by Lab View human-machine control interface. Finally a generic device LIV measurement by the standard probe station and Kiethley current source will demonstrate superior power output and efficient droop in our NPLED device. III. R ESULTS AND D ISCUSSION First step to compare these two material growth methods is to check their material quality. In order to analyze the detailed epitaxial layer quality, we used TEM to compare the cross section between two types of devices in Fig. 5. A comparison of Fig. 5(a) and 5(b) shows that the dislocation density in the NPSi sample is reduced much more than that of MPSi’s. The TDDs for MPSi is estimated to be 2.5 × 1010 cm−2 at the bottom of the n-GaN layer, and it decreases to 4.6 × 109 cm−2 at the top of the n-GaN layer and 6.2 × 108 cm−2 in the p-GaN

Fig. 5. TEM images of LEDs grown on (a) MPSi and (b) NPSi; (c) and (d) region of between AlGaN layer and Si substrate for NPSi using g = (0002).

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Fig. 6. Top view CL images on samples of energy for (a) micro-scale and (b) nano-scale pattered Si substrate.

region. On the other hand, for the epilayer grown on NPSi, fewer dislocations are observable within the range of view. As shown in Fig. 5(b), the TDDs at the bottom of the n-GaN layer is about 1.1 × 1010 cm−2 ; however, the TDDs at the top of the n-GaN layer drop down to 5.7 × 108 cm−2 , and it is only 8.8 × 107 cm−2 in the p-GaN region. The reduction of TDDs NPSi over MPSi is about 10 times. Fig. 5(c) and 5(d) are TEM images are taken at the interface of epilayer/NPSi. As can be seen in Fig. 5(c), there are many dislocations bent and terminated in AlGaN layer or near the epilayer/NPSi interface. As a result, the density of TDDs in the subsequent quantum well region was much lower. Next, we will examine our results by CL. CL is a very important technique when we need non-invasive assessment of crystal quality. Fig. 6(a) and 6(b) display the plan-view CL emission images with a 10 kV accelerating voltage at room temperature. At first glance, MPLED showed more “dead zone” or black spots than NPLED. These dark areas in the CL images are regions where minority carriers get consumed by dislocations due to high non radiative recombination velocity [20]. The other feature we would like to point out is that the emission intensity of MPLED is less uniform than NPLED’s. This was mainly due to indium composition fluctuation and the

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phase separation [21], [22]. These CL images suggest that the pitch between the etched silicon holes might play an important role since the nano-patterned sample looks much better. To further investigate how the pitch of nano-patterns affects the photon emission efficiency, we cleaved through nano-porous wafers and performed the cross section CL measurement. The upper half of Fig. 7(a) shows the cross section CL intensity of NPLEDs’ quantum well region, and it is taken at the same horizontal location aligned to the nano-scale patterned Si/GaN substrate underneath (bottom half of Fig. 7(a)). We noticed that CL intensity is much stronger when etched silicon holes are closer. To quantitatively evaluate this observation, we plot the average intensity between silicon holes against interpore distances in Fig. 7(b). When the interpore distance reduced to 0.2μm (200nm) or less, the integrated luminescence intensity grows sharply. From previous research by Sugahara, et. al. [23], the CL efficiency (η) can be related to sample recombination behavior given by: Ld 2 8 r − r0 2r0 2 − 2 r exp − dr (2) η =1− Ld Lp L d r0 where Ld is the mean dislocation distance, L is the diffusion length in InGaN, and r0 is the radius in which non-radiative recombination consumes all carriers (the dark spot). If other material characteristics is the same and assume uniform excitation, the only factor that can affect the luminescent intensity is Ld , the mean dislocation distance. So when material has fewer defects, the efficiency is higher. From the trend of data, we can reasonably conclude that the higher density of nano-

size interpore area bears less dislocation and thus tends to have strong light emission. Just like CL can reveal the crystal quality, PL can let us find out the possible radiative recombination mechanism in the quantum well region. It has been shown that thermal quenching of PL intensity can be explained by carriers’ thermal emission out of a confining potential with an activation energy correlated with the depth of the confining potential [24]. Therefore, it is expected that the deeper localization with better confinement should have larger activation energy. Fig. 8(a) and 8(b) display the temperature dependence of PL intensity fitted by Arrhenius equation as following [25]: I (T ) =

I0

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+ B exp

−E b kB T

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where I(T) is the temperature-dependent PL intensity, I0 is the PL intensity at 20 K, kB is Boltzmann’s constant, A and B are the rate constants, and Ea and Eb are the activation energies for two different nonradiative channels, which correspond to the low temperature and high temperature regions [26]. For high temperature region, thermal quenching can be fitted with activation energy (Eb ) 59 and 87 meV for MPLEDs and NPLEDs, respectively. In particular, the activation energy for NPLEDs is 47.4 % higher than that for MPLEDs, leading to a minor overflow of carriers outside the InGaN MQW active region. The discrepancy should rise from either anisotropic distribution in the active region or mixture of thermionic emission from potential minimum to barrier. Based on above result, the PL-intensity improvement in the NPLEDs can be attributed to the stronger localization effects and better carrier confinement in In0.08 Ga0.92 N/GaN MQW active region [27]. Potential variation affects how easy the carrier can be confined, and the combining rate can be regarded as how fast the carriers can recombined. The information about carrier recombination rate can be obtained from decaying behavior of photoluminescence. The low temperature TRPL decay for both samples was shown in Fig. 9. Because the measurement was carried out at 10K, the influence of the nonradiative recombination process could be excluded [28]. The TRPL results can be fitted by a bi exponential decaying function: [29] t t + I2 (0) exp − (4) I (t) = I1 (0) exp − τ1 τ2 where I(t) is the PL intensity at time t; τ1 and τ2 represent the characteristic lifetimes of the carriers. The fast decay time


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Fig. 10. Forward I–V characteristics of all fabricated LEDs, and the inset is reverse I–V characteristics of all fabricated LEDs.

constant (τ1 ) usually represents the radiative recombination of excitons and the relaxation of QW excitons from free or extended states toward localized states [29], [30]. Our fitting shows τ1 = 3.2 and 1 ns for MPLEDs and NPLEDs, respectively. The slow decay time (τ2 ) accounts for communication between localized states and localized excitons [29], [30]. The fitting shows τ2 = 9.4 and 3.2 ns for MPLEDs and NPLEDs, respectively. In both fast and slow constants, NPLEDs’ lifetime is generally shorter than MPLEDs’ at low temperature. S. Chichibu, et. al. reported the electron-hole pairs in the potential minima of QWs can be referred to as localized excitons, and the emission efficiency can still be enhanced even though the wave function overlap is weakened [31]. In the case of MPLEDs and NPLEDs, much higher radiative recombination rate observed in TRPL can be interpreted as direct evidence of stronger localized confinement in NPLEDs than MPLEDs, and also an indication of more efficient lightemitter. The final trial of this nano-size template is to test the light emitting efficiency from the real device. LED devices with a chip size of 350 × 350 μm2 were fabricated on both MPLEDs and NPLEDs. Fig. 10 shows the forward I-V characteristics of both samples. At 20 mA forward current, both samples exhibited diode voltages around 4.7 V. In addition, at the

reverse bias (shown in the insert plot of Fig. 10), the leakage current of the NPLEDs is smaller than MPLEDs. Several types of dislocations can contribute to the reverse-bias leakage current [32], and one of the most dominant type is the screw dislocation [32], [33]. The reduction of screw type dislocations can certainly help to reduce the reverse-bias current, and our measurement indicates a better crystal quality of LEDs grown on NPSi substrate, which confirms with TEM results. Fig. 11 shows EL spectrums as a function of injection current for MPLEDs and NPLEDs. The emission peak wavelength of NPLEDs is slightly shorter than MPLEDs’. In our previous study, we performed Raman backscattering measurement at room temperature. The regular Raman shift of E2 (High) in stress-free GaN layer is around 567.2 cm−1 . In this paper, the E2 (High) shift is 565.4 cm−1 and 564.5 cm−1 , for samples on NPSi and on MPSi, respectively. The deviation of the E2 (High) peaks from the intrinsic position is proportional to the residual tensile stress. For GaN, the E2 (High) mode shifts linearly with stress in 2.9 cm−1 /GPa for biaxial stress. We can thus estimate the tensile stress in NPSi and MPSi are 0.62 GPa and 0.93 GPa, respectively. This indicates that the LEDs grown on NPSi exhibited lower strain than on MPSi. Therefore, the LEDs grown on NPSi possess a reduced QCSE. The related MPSi and NPSi Raman measurement results have been previously published by Dongmei Deng et. al. [18]. Moreover, we can see EL emission peak wavelength of MPLEDs exhibits blue shift from 429 nm to 427 nm with increasing drive current as shown in Fig. 11(a). However, we obtained almost unshifted EL peak with increasing injection current. This result indicates that the quantum confined stark effect (QCSE) does become weaker due to the strain relaxation in epitaxial layer overgrown on NPSi template [34]. Finally, Fig. 12 shows the light output intensity and normalized external quantum efficiency (EQE) as a function of forward current density for both samples. The light output-

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current curve of MPLEDs is linear under 20mA/cm2. From this data, the light output of the MPLED shows an obvious soft increase of EQE, which is an indication of higher threading dislocation densities [35], and the roll-over current is much lower in MPLED. The data from NPLED, however, demonstrates good material quality with a reduced efficiency droop and much less soft increase in EQE. However, it rolls over beyond 20mA/cm2 with a reduced EQE. The EQE is decreased to 62% of its maximum value when the current density at 100mA/cm2. In contrast, the NPLEDs exhibits 20% efficiency droop with increasing the injection current density to 100mA/cm2. It can be attributed to reduced polarization field which also echoes to weaker QCSE under the circumstance of reduced strain in overgrown layers on NPSi template [36]. IV. C ONCLUSION In conclusion, the optical and electrical properties of LEDs grown on micro and nano-scale patterned Si substrate were investigated. We demonstrated a more homogeneous growth of InGaN/GaN active layers under this nano-scale template by plan-view and cross-section CL mapping. From temperature dependent PL and low temperature TRPL measurement, NPLEDs has better carrier confinement and higher radiative recombination rate than MPLEDs. On the actual device performance, NPLEDs exhibits smaller peak wavelength blue shift, lower reverse leakage current and decreases efficiency droop compared with the MPLEDs. The results suggest a weaker QCSE due to relaxation of strain in the epitaxial layers on nano-scale patterned substrate, which can be really useful for the next generation of large area, Si-based heteroepitaxy of GaN related optoelectronic devices. ACKNOWLEDGMENT The authors would like to thank S. C. Wang of National Chiao-Tung University, Hsinchu, Taiwan, for useful discussion. R EFERENCES [1] E. F. Schubert, Light Emitting Diodes, 2nd ed. Cambridge, U.K.: Cambridge Univ. Press, 2003, pp. 21–22. [2] J. Han, M. H. Crawford, R. J. Shul, J. J. Figiel, M. Banas, L. Zhang, Y. K. Song, H. Zhou, and A. V. Nurmikko, “AlGaN/GaN quantum well ultraviolet light emitting diodes,” Appl. Phys. Lett., vol. 73, no. 12, pp. 1688–1690, Sep. 1998. [3] S. Nakamura, S. Pearton, and G. Fasol, “The blue laser diode,” in A Complete Story, 2nd ed. Berlin, Germany: Springer-Verlag, 2000, p. 48. [4] S. Guha and N. A. Bojarczuk, “Ultraviolet and violet GaN light emitting diodes on silicon,” Appl. Phys. Lett., vol. 72, no. 4, pp. 415–417, Jan. 1998. [5] C. A. Tran, A. Osinski, R. F. Karlicek, and I. Berishev, “Growth of InGaN/GaN multiple-quantum-well blue light-emitting diodes on silicon by metalorganic vapor phase epitaxy,” Appl. Phys. Lett., vol. 75, no. 11, pp. 1494–1496, Sep. 1999. [6] S. Dalmasso, E. Feltin, P. de Mierry, B. Beaumont, P. Gibart, and M. Leroux, “Green electroluminescent (Ga, In, Al)N LEDs grown on Si (111),” Electron. Lett., vol. 36, no. 20, pp. 1728–1730, Sep. 2000. [7] M. A. Sánchez-García, F. B. Naranjo, J. L. Pau, A. Jiménez, E. Calleja, and E. Muñoz, “Ultraviolet electroluminescence in GaN/AlGaN singleheterojunction light-emitting diodes grown on Si(111),” J. Appl. Phys., vol. 87, no. 3, pp. 1569–1571, 2000.

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[29] W. Z. Lee, G. W. Shu, J. S. Wang, J. L. Shen, C. A. Lin, W. H. Chang, R. C. Ruaan, W. C. Chou, C. H. Lu, and Y. C. Lee, “Recombination dynamics of luminescence in colloidal CdSe/ZnS quantum dots,” Nanotechnology, vol. 16, no. 9, pp. 1517–1521, 2005. [30] T. S. Ko, T. C. Lu, T. C. Wang, J. R. Chen, R. C. Gao, M. H. Lo, H. C. Kuo, S. C. Wang, and J. L. Shen, “Optical study of a-plane InGaN/GaN multiple quantum wells with different well widths grown by metal-organic chemical vapor deposition,” J. Appl. Phys., vol. 104, no. 9, pp. 093106-1–093106-8, Nov. 2008. [31] S. Chichibu, T. Sota, K. Wada, and S. Nakamura, “Exciton localization in InGaN quantum well devices,” J. Vac. Sci. Technol. B.: Microelectron. Nanometer Struct., vol. 16, no. 4, pp. 2204–2214, Jul. 1998. [32] E. J. Miller, E. T. Yu, P. Waltereit, and J. S. Speck, “Analysis of reversebias leakage current mechanisms in GaN grown by molecular-beam epitaxy,” Appl. Phys. Lett., vol. 84, no. 4, pp. 535–537, Jan. 2004. [33] E. G. Brazel, M. A. Chin, and V. Narayanamurti, “Direct observation of localized high current densities in GaN films,” Appl. Phys. Lett., vol. 74, no. 16, pp. 2367–2369, Apr. 1999. [34] C.-H. Chiu, P.-M. Tu, C.-C. Lin, D.-W. Lin, Z.-Y. Li, K.-L. Chuang, J.-R. Chang, T.-C. Lu, H.-W. Zan, C.-Y. Chen, H.-C. Kuo, S.-C. Wang, C.-Y. Chang, C.-H. Chiu, P.-M. Tu, D.-W. Lin, Z.-Y. Li, T.-C. Lu, H.W. Zan, H.-C. Kuo, and S.-C. Wang, “Highly efficient and bright LEDs overgrown on GaN nanopillar substrates,” IEEE J. Sel. Topics Quantum Electron., vol. PP, no. 99, pp. 1–8, 2010. [35] M. F. Schubert, S. Chhajed, J. K. Kim, and E. F. Schubert, D. D. Koleske, M. H. Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, “Effect of dislocation density on efficiency droop in GaInN/GaN light-emitting diodes,” Appl. Phys. Lett., vol. 91, no. 23, pp. 231114-1– 231114-3, Dec. 2007. [36] Y. B. Tao, Z. Z. Chen, F. F. Zhang, C. Y. Jia, S. L. Qi, T. J. Yu, X. N. Kang, Z. J. Yang, L. P. You, D. P. Yu, and G. Y. Zhang, “Polarization modification in InGaN/GaN multiple quantum wells by symmetrical thin low temperature-GaN layers,” J. Appl. Phys., vol. 107, no. 10, pp. 103529-1–103529-5, May 2010.

Ching-Hsueh Chiu was born in Hsinchu, Taiwan, in 1983. He received the B.S. degree in physics from the Chung Yuan Christian University, Chung-Li, Taiwan, in 2006, and the M.S. degree in electrophysics from the National Chiao Tung University (NCTU), Hsinchu, in 2008. He is currently pursuing the Ph.D. degree in the Department of Photonics, NCTU. He joined the Semiconductor Laser Technology Laboratory, NCTU, in July 2008. His current research interests include III–V compound semiconductor materials growth by metal organic chemical vapor deposition and characteristic study under the instruction Prof. H.-C. Kuo and T.-C. Lu.

Chien-Chung Lin was born in Taipei, Taiwan, in 1970. He received the B.S. degree in electrical engineering from the National Taiwan University, Taipei, in 1993, and the M.S. and Ph.D. degrees in electrical engineering from Stanford University, Stanford, CA, in 1997 and 2002, respectively. His thesis work focused on design, modeling and fabrication of micromachined tunable optoelectronic devices. He has been with the National Chiao-Tung University (NCTU), Tainan, Taiwan, since 2009, where he holds a position as an Assistant Professor. The major research efforts in his group are in design and fabrication of semiconductor optoelectronic devices, including light emitting diodes, solar cells and lasers. Before joining NCTU, he was with different start-ups in the United States. After graduating from Stanford in 2002, he joined E2O Communications, Inc., Calabasas, CA, as a Senior Optoelectronic Engineer. In 2004, he joined Santur Corporation, Fremont, CA, where he initially worked as a Member of Technical Staff then became a Manager of laser chip engineering. He had worked on various projects such as monolithic multi-wavelength distributed feedback (DFB) laser arrays for data and telecommunications applications and yield and reliability

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analysis of DFB laser arrays. He has more than 30 journal and conference publications. His current research interests include optically and electrically pumped long-wavelength vertical cavity surface emitting lasers. Dr. Lin is a member of the IEEE Photonic Society and the Electron Devices Society.

Dong-Mei Deng received the Graduate degree with the Doctor degree in electronic and computer engineering at the Hong Kong University of Science and Technology, Kowloon, Hong Kong, in 2010, with a research background in metalorganic chemical vapor deposition growth. She is currently with the Hong Kong Applied Science and Technology Research Institute, Shatin, Hong Kong.

Da-Wei Lin received the B.S. degree in photonics and the M.S. degree in electro-optical engineering from the National Chiao Tung University, Hsinchu, Taiwan, in 2009 and 2010, respectively. He is currently pursuing the Ph.D. degree in the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu. His current research interests include optical measurement and analysis and nano-structure analysis for GaN-based light emitting diodes.

Jin-Chai Li received the B.S. degree in physics and the Ph.D. degree in microelectronics and solid state electronics from Xiamen University, Xiamen, China, in 2002 and 2008, respectively. She joined Prof. S.C. Wang’s Group in the Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, in 2009, as a PostDoctoral Research Fellow. Her current research interests include first principle simulation and epitaxial growth of III–V materials and optoelectronic devices.

Zhen-Yu Li was born in Chiayi, Taiwan, on October 2, 1980. He received the B.S. degree from the Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li, Taiwan, in 2003, and the Ph.D. degree in engineering from Chung Yuan Christian University in 2007. He joined Prof. S.C. Wang’s Group in the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan, in October 2007, as a Post-Doctoral Research Fellow. His current research interests include metal organic chemical vapor deposition heteroepitaxial growth, process and characterization of optoelectronic devices, such as vertical cavity surface emitting lasers, resonant cavity light emitting diodes and solar cells.

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Gia-Wei Shu received the Ph.D. degree in physics from Chung Yuan Christian University, Chung-Li, Taiwan, in 2007. He was an Engineer in the Solar Cells Department, Solapoint Corporation, Hsinchu, Taiwan, in 2008. He is currently a Post-Doctoral Researcher at Chung Yuan Christina University. His current research interests include bandgap engineering of semiconductors and time-resolved phenomena in nanomaterials.

Tien-Chang Lu received the B.S. degree in electrical engineering from the National Taiwan University, Taipei, Taiwan, in 1995, the M.S. degree in electrical engineering from the University of Southern California, Los Angeles, in 1998, and the Ph.D. degree in electrical engineering and computer science from the National Chiao Tung University, Hsinchu, Taiwan, in 2004. He joined the Department of Photonics, National Chiao Tung University, as a faculty member in August 2005. He has authored or co-authored more than 100 international journal papers. His current research interests include design epitaxial growth, process and characterization of optoelectronic devices, such as Fabry-Perot type semiconductor lasers, vertical cavity surface emitting lasers, resonant cavity light emitting diodes (LEDs), microcavities, photonic crystal surface emitting lasers, wafer-fused flip-chip LEDs and solar cells. Prof. Lu is a recipient of the Exploration Research Award of Pan Wen Yuan Foundation in 2007 and the Excellent Young Electronic Engineer Award in 2008.

Ji-Lin Shen received the Ph.D. degree in physics from the National Taiwan University, Taipei, Taiwan, in 1994. He completed post-doctoral research in the Electrical Engineering Department, University of California, Los Angeles, then was an Assistant Professor at Chung Yuan Christian University, ChungLi, Taiwan, in 1998. He is currently a Professor in the Physics Department, Chung Yuan Christian University. His current research interests include optical characterization of semiconductor materials and nano-materials.

Hao-Chung Kuo (M’98–SM’06) received the B.S. degree in physics from the National Taiwan University, Taipei, Taiwan, the M.S. degree in electrical and computer engineering from Rutgers University, New Brunswick, NJ, in 1995, and the Ph.D. degree from the Electrical and Computer Engineering Department, University of Illinois at Urbana-Champaign, Urbana, in 1998. He has an extensive professional career both in research and industrial research institutions that includes Research Assistant in Lucent Technologies, Bell Laboratories, Murray Hill, NJ, from 1993 to 1995, and a Senior Research and Development Engineer in the Fiber-Optics Division, Agilent Technologies, Santa Clara, CA, from 1999 to 2001, and LuxNet Corporation, Fremont,

CA, from 2001 to 2002. Since October 2002, he has been with the Institute of Electro-Optical Engineering, National Chiao Tung University (NCTU), Hsinchu, Taiwan, as a faculty member. He was the Associate Dean of the Office of International Affairs, NCTU, from 2008 to 2009, and has been a Director of the Institute of Electro-Optical Engineering since August 2009. He has authored or co-authored 200 internal journal papers, two invited book chapters, six granted and 10 pending patents. His current research interests include semiconductor lasers, vertical cavity surface-emitting lasers, blue and ultraviolet light-emitting diode lasers, quantum-confined optoelectronic structures, optoelectronic materials, and solar cells. Prof. Kuo is an Associate Editor of the IEEE/Optical Society of America (OSA) J OURNAL OF L IGHTWAVE T ECHNOLOGY and J OURNAL OF S ELECTED T OPICS IN Q UANTUM E LECTRONICS —Special Issue on solid state lighting in 2009. He received the Ta-You Wu Young Scholar Award from the National Science Council Taiwan in 2007 and the Young Photonics Researcher Award from the OSA/Society of Photo-Optical Instrumentation Engineers Taipei Chapter in 2007.

Kei May Lau (S’78–M’80–SM’92–F’01) received the B.S. and M.S. degrees in physics from the University of Minnesota, Minneapolis, and the Ph.D. degree in electrical engineering from Rice University, Houston, Texas. She started as a Senior Engineer at M/A-COM Gallium Arsenide Products, Inc., Lowell, MA, where she worked on epitaxial growth of GaAs for microwave devices, development of high-efficiency and mm-wave impact ionization avalanche transittime diodes, and multi-wafer epitaxy by the chloride transport process. After two years in industry, she joined the faculty of the Electrical and Computer Engineering Department, University of Massachusetts (UMass), Amherst, where she became a Professor in 1993. She initiated metalorganic chemical vapor deposition, compound semiconductor materials and devices programs at UMass. Her research group has performed studies on heterostructures, quantum wells, strained-layers, III–V selective epitaxy, high-frequency and photonic devices. She spent her first sabbatical leave at Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, and was with the Electro-Optical Devices Group. She developed acoustic sensors at the DuPont Central Research and Development Laboratory, Wilmington, DE, during her second sabbatical leave. She was a Visiting Professor at Hong Kong University of Science and Technology (HKUST), Kowloon, Hong Kong, in 1998. She has been a Chair Professor/Professor in the Electronic and Computer Engineering Department at HKUST since 2000. Her current research interests include metamorphic growth of III–V devices on silicon substrates. Prof. Lau is a recipient of the National Science Foundation Faculty Awards for Women Scientists and Engineers in 1995 and Croucher Senior Research Fellowship in 2008. She served on the IEEE Electron Devices Society Administrative Committee and was an Editor of the IEEE T RANSACTIONS ON E LECTRON D EVICES from 1996 to 2002. She also served on the Electronic Materials Committee of the Minerals, Metals and Materials Society of American Institute of Materials Engineers, and was an Editor of the Journal of Crystal Growth.